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A species of weta is now considered "nationally critical" - the most serious category for threatened wildlife - while another has been reclassified as "nationally vulnerable" because of redback spiders invading its territory.

The Department of Conservation has published a new stocktake of New Zealand's more than 170 known orthoptera insects, including weta, crickets and grasshoppers.

While most were considered either "not threatened" (50 per cent), "naturally uncommon" (18 per cent) or "data deficient" (16 per cent), eight species were considered threatened, including two "nationally critical" (the Hemiandrus "furoviarus" ground weta and Sigaus homerensis alpine grasshopper), two nationally endangered and four nationally vulnerable.

The Hemiandrus "furoviarius", found in Tekapo riverbeds, was reclassified to nationally critical because of observed decline, while the Hemiandrus "Cromwell" moved from "naturally uncommon" to "nationally vulnerable" as a result of reassessment of the likely impacts of the introduced Australian redback spiders now abundant in their habitat.

An earlier Enviroment Canterbury report described the Hemiandrus "furoviarius", also called the Tekapo ground weta, as being known only from river margins around the Mackenzie Basin, where it burrowed in silty soils.

It usually preferred terraces above normal flows and small floods, but was susceptible to severe flooding.

Massey University ecologist Professor Steve Trewick, a co-author of the report, said researchers will still working to understand the distribution and abundance of many invertebrate groups, including orthoptera.

Most species of so-called tree weta were faring comparatively better, perhaps because they could breed fast enough and were good enough to escape predators to maintain populations, even in urban environments.

Researchers were trying to learn more about new and already identified species, either by demonstrating that they represented populations of existing species or by formally describing them as new ones.

"As that work proceeds however, some bits of ecological information are being gleaned about specific populations," Trewick said.

"Inevitably, small amounts of information and observation tend to attract a lot of attention because of the dearth of information about these animals in general."

"That said, in some regions of the country that we know are highly modified, and continue to be modified by direct human activity or invasion of introduced species - especially central Otago/Canterbury with irrigation, mining, weed invasion and Australian spiders - it is very likely that there are local species of orthoptera, including [ground weta], that are threatened."

The status of the native grasshoppers, most of which were found in subalpine habitats, were faring generally well, but populations of those found in lowland areas were declining to and would continue to as activities like dairying and irrigation expanded into their habitats.

"On the whole, lowland populations have been lost."

The other major orthoptera family - Rhaphidophoridae or cave weta - included many undescribed species with unknown ranges.

"Lots of small species inhabit forest and we are still discovering new species and trying to piece together their ecological requirements," Trewick said.

"This group in particular is going to need to be revised extensively and this will result in an increase in the number of species, although it is likely that the taxa we know least about are the ones that are scarcest and in areas that are most modified."